Image display method for liquid crystal mask laser marker

ABSTRACT

An image display method for a liquid crystal mask marker displays an image of a high sharpness even when the marking speed is high. To this end, when the display image of a liquid crystal mask (4) is to be rewritten, the method includes performing at least one of (a) making all of the opaque picture elements (B) of the current image (P1) to become transparent, and (b) making all of the transparent picture elements (A) of the current image (P1) to become opaque; then making the electrodes of all of the picture elements (A,B) to be grounded or set equipotentially; and then starting the display of the next image (P2).

TECHNICAL FIELD

The present invention relates to an image display method for a liquidcrystal mask marker, and more particularly to an image display methodfor a liquid crystal mask marker which can clearly display and imprintmask images, such as product numbers and patterns, on the surfaces ofnonmetallic articles, such as IC chips, or metallic articles.

BACKGROUND ART

A prior art liquid crystal mask marker will be described with referenceto FIG. 5. A high-density energy light beam R1 emitted from a laseroscillator 1, for example, is irradiated onto a display image of aliquid crystal mask 4, and a light beam R2 transmitted through the mask4 is then irradiated onto a workpiece surface 6 for imprinting the imageon the workpiece surface 6. According to Japanese Patent Laid-Open No.5-210358, for example, the display image of the liquid crystal mask 4 isrewritten with a controller 8 in such a manner that, as shown in FIG.6B, potentials of all of the picture elements are grounded as indicatedby S, i.e., voltages applied to all of the picture elements are madezero, and the step of displaying the next display image P is thenstarted (this rewriting process will be hereinafter referred to as"process of grounding potentials of all of the picture elements").

With the process of grounding potentials of all of the picture elements,there occur phenomena, called an afterimage and a counter afterimagedescribed later, in the display image of the liquid crystal mask 4immediately after the rewriting. Referring to FIGS. 7A to 7C, forexample, when a solid black circle A (FIG. 7A) is marked on theworkpiece surface 6 and, after rewriting the display image of the liquidcrystal mask 4, a solid black rectangle B (FIG. 7B) is marked on theworkpiece surface 6, a rectangle C appears, together with an afterimageA2, as the image of the liquid crystal mask 4 immediately after therewriting, as shown in FIG. 7C. In addition, the rectangle C is dividedinto a clear area A1 and an unclear area B1. This unclear area B1 isdefined here as "counter afterimage". Incidentally, the letter Bindicates an area of the image other than the circle A.

The afterimage A2 and the counter afterimage B1 will disappear naturallywith the elapse of time. Therefore, those phenomena are practically notproblematic in conventional liquid crystal mask markers which are muchinferior in marking speed and image sharpness to the present liquidcrystal mask marker described later. More specifically, in conventionalliquid crystal mask markers, the high-density energy light beam R1 isirradiated onto the image including both the afterimage A2 and thecounter afterimage B1. But when the image is imprinted on the workpiecesurface, the counter afterimage B1 does not raise a problem because theirradiation time is long. On the other hand, the afterimage A1 does notappear as imprinted on the workpiece surface by irradiating thehigh-density energy light beam R1 onto the image in which the afterimageA2 has disappeared. It is to be noted that the counter afterimage B1will not also appear as imprinted on the workpiece surface if thehigh-density energy light beam R1 is, as with the above case of theafterimage A2, irradiated onto the image in which the counter afterimageB1 has disappeared. In practice, however, no considerations have beenpaid so far to the foregoing point and the level of sharpness of theimage imprinted does not yet deserve discussing in the present state ofart.

Meanwhile, the applicant has previously proposed several liquid crystalmask markers (see, e.g., Japanese Patent Laid-Open No. 5-42379) whereinthe rewriting time and the image display time are each on the order ofabout 30 msec at the minimum, i.e., an image is displayed and rewrittenat very high speeds, while ensuring a sharpness of the image imprinted.Accordingly, the irradiation time of a high-density energy light beam(i.e., the marking time) during the image display time is even shorter.In the proposed liquid crystal mask markers, not only the afterimage A2and the counter afterimage B1, but also the timing at which thehigh-density energy light beam R1 is irradiated onto an image, bringabout a great effect on achievement of a high marking speed and a highimage sharpness.

Thus, such a liquid crystal mask marker with a high marking speed and ahigh image sharpness causes a problem as follows. If the processing ofan afterimage, the processing of a counter afterimage, and theirradiation timing of a high-density energy light beam areinappropriate, the afterimage A2 and the counter afterimage B1 as shownin FIG. 7C would appear distinctly as imprinted on the workpiece surfacebecause of a capability of the marker to display an image of a highsharpness.

SUMMARY OF THE INVENTION

The present invention has been accomplished with a view of solving theabove-stated problem in the prior art, and its object is to provide animage display method for a liquid crystal mask marker which can displayan image of a higher sharpness at a higher speed with more reliability,taking into account the processing of an afterimage, the processing of acounter afterimage, and the irradiation timing of a high-density energylight beam.

According to a first invention of this application, an image displaymethod for a liquid crystal mask marker is featured in that when adisplay image of a liquid crystal mask is rewritten, all of the pictureelements of a current image are made transparent, then the electrodes ofall of the picture elements are grounded or set equipotentially, andthereafter the step of displaying a next image is started.

According to a second invention of this application, an image displaymethod for a liquid crystal mask marker is featured in that when adisplay image of a liquid crystal mask is rewritten, all of the pictureelements of a current image are made opaque, then the electrodes of allof the picture elements are grounded or set equipotentially, andthereafter the step of displaying a next image is started.

According to a third invention of this application, an image displaymethod for a liquid crystal mask marker is featured in that when adisplay image of a liquid crystal mask is rewritten, the states of thepicture elements of a current image are reversed such that transparentpicture elements are made opaque and opaque picture elements are madetransparent, then the electrodes of all of the picture elements aregrounded or set equipotentially after the above reversing step, andthereafter the step of displaying a next image is started.

Prior to describing the operation of the foregoing first to thirdinventions, a description will be first made of the results of analysismade by the inventors on the above-stated problem in the prior art. InFIG. 4 which is an analytic graph for the prior art, the referencenumerals in FIGS. 7a to 7c are also employed. It is here assumed that acurrent image P1, comprising a transparent area A and an opaque area B,is rewritten with the prior art process of grounding the potentials ofall of the picture elements and the step of displaying a next image P2is then started. The next image P2 has a transparent area comprising oneportion A1 of the area A and one portion B1 of the area B, and an opaquearea comprising the other portion A2 of the area A and the other portionB2 of the area B. FIG. 4 is a correlative graph between thetransmittance of each picture element in A, B, A1, A2, B1, and B2 andthe time in the steps of displaying the current image P1, rewriting theimage, and displaying the next image P2. The graph of FIG. 4 alsoindicates the timing Ts to start the irradiation by a high-densityenergy light beam and the irradiation time T. The sharpness of an imageimprinted is represented by the product of the intensity of a light beamitself transmitted through each picture element and the transmissiontime "t" of the light beam. The transmission time "t" is equal to theirradiation time T (t=T) when a liquid crystal mask marker is of thelump irradiating type, but is given as time (t=T/v) resulted by dividingthe irradiation time T by the raster speed "v" when it is of the rasterirradiating type.

When the current image P1 is rewritten to the next image P2 with theprior art process of grounding the potentials of all of the pictureelements, as shown in FIG. 4, the transmittance of the picture elementB1 rises later than that of the picture element A1, while thetransmittance of the picture element A2 falls later than that of thepicture element B2. In other words, such a delay in the rising of thetransmittance of the picture element B1 gives rise to a counterafterimage B1, while such a delay in the falling of the transmittance ofthe picture element A2 give rise to an afterimage A2. Because ahigh-density energy light beam is irradiated onto the next image P2without taking into account the foregoing phenomena, the afterimage A2and the counter afterimage B1 are imprinted on the workpiece surface 6.In addition, as the marking speed increases, the timing Ts to start theirradiation comes closer to the period of rewriting and, therefore, thesharpness of an image imprinted reduces more remarkably due to theoccurrence of the afterimage A2 and the counter afterimage B1.

Another new problem can also be seen from FIG. 4. More specifically,when a liquid crystal mask marker is of the lump irradiating type, theafterimage A2 and the counter afterimage B1 occur uniformly. However,when a liquid crystal mask marker is of the raster irradiating type, theafterimage A2 and the counter afterimage B1 are more distinct in thepicture elements of the next image P2 on the side where the rasterirradiation is started, than in the picture elements thereof on the sidewhere the raster irradiation is ended. Further, tendency analysis forthe state of each picture element shows that if the picture element isin a stable state regardless of whether its past state was transparentor opaque, it takes a relatively long time to transit the pictureelement to the other state. Stated otherwise, if the history of thepicture element is unstable or if the picture element is made unstableby cancelling its history, the transition to the other state takes arelatively short time. The image display methods for the liquid crystalmask marker according to the first to third inventions have beenachieved based on the above results of analysis.

In the feature of the first invention, the expression "are madetransparent" means that both the picture elements A and B, i.e., all ofthe picture elements, of the current image are turned once to a statetransparent to the irradiated light beam. In practice, since the pictureelements A of the current image P1 are already in the transparent state,the voltage for only the picture elements B is raised to a value enoughto make the picture elements B transparent. Also, the expression"electrodes of all of the picture elements are grounded" means that allof the picture elements are processed in the same manner as the priorart process of grounding the potentials of all of the picture elements.Thus, according to the feature of the first invention, the history ofeach picture element B in the current image P1 is cancelled, thenelectrodes of all of the picture elements are grounded or setequipotentially, and thereafter the step of displaying the next image P2is started.

In the feature of the second invention, the expression "are made opaque"means that both the picture elements A and B, i.e., all of the pictureelements, of the current image are turned once to a state opaque to theirradiated light beam. In practice, since the picture elements B of thecurrent image P1 are already in the opaque state, the voltage for onlythe picture elements A is lowered to a value enough to make the pictureelements A opaque. Thus, according to the feature of the secondinvention, the history of each of the picture elements in the currentimage P1 is cancelled, then the electrodes of all of the pictureelements are grounded or set equipotentially, and thereafter the step ofdisplaying the next image P2 is started.

In the feature of the third invention, the expression "are reversed"means that the voltage for the picture elements A is lowered and thevoltage for the picture elements B is raised. At this time, it is notnecessarily required to turn the picture elements A from the transparentstate to the completely opaque state and turn the picture elements Bfrom the opaque state to the completely transparent state. It isessential that the voltages are merely raised and lowered correspondingto the respective picture elements. Thus, according to the feature ofthe third invention, the history of each of the picture elements A and Bin the current image P1 is cancelled, then the electrodes of all of thepicture elements are grounded or set equipotentially, and thereafter thestep of displaying the next image P2 is started.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 plot changes in transmittance of light through images andafterimages before and after the rewriting of a display image, ascarried out according to the present invention, in which:

FIG. 1 is an analytic graph for a first embodiment,

FIG. 2 is an analytic graph for a second embodiment, and

FIG. 3 is an analytic graph for a third embodiment,

FIG. 4 is an analytic graph plotting changes in transmittance of lightthrough images and afterimages before and after a rewriting of a displayimage, as carried out according to the prior art,

FIG. 5 is a diagram of a liquid crystal mask marker according to anembodiment of the present invention,

FIGS. 6A and 6B illustrate procedures for rewriting a display image of aliquid crystal mask, in which: FIG. 6A is an explanatory view for theprocedures for each of the workpieces, and FIG. 6B is an explanatoryview for the procedures for each of the divided image segments withinone period of marking time shown in FIG. 6A, and

FIGS. 7A to 7C illustrate, for the purpose of explaining a sharpness ofthe image imprinted, images displayed on the liquid crystal mask and onthe workpiece surface, in which: FIG. 7A is an explanatory view of acurrent image, FIG. 7B is an explanatory view of a next image, and FIG.7C is an explanatory view of the next image including an afterimage anda counter afterimage.

BEST MODE FOR CARRYING OUT THE INVENTION

Several preferred embodiments of an image display method for a liquidcrystal mask marker according to the present invention will behereinafter described in detail with reference to the accompanyingdrawings.

FIG. 5 shows a liquid crystal mask marker employed in the firstinvention. The system arrangement of this marker will be describedbriefly. A laser beam R1, emitted from a laser oscillator 1, isirradiated in a raster manner onto the image surface of a liquid crystalmask 4 through a deflector 2 for raster irradiation. The term "rasterirradiation" means that the thin laser beam R1 is irradiated while beingmoved successively in the X and Y directions. A laser beam R2,transmitted through the image surface of the liquid crystal mask 4, isthen irradiated onto a workpiece surface 6 through a deflector 5. Thetransmitted laser beam R2 contains information of a mask display image,and this image is imprinted on the workpiece surface 6. The transmittedlaser beam R2 is condensed toward the deflector 5 through the liquidcrystal mask 4 by a condensing lens 3, placed upstream of the liquidcrystal mask 4.

In the illustrated system example of a liquid crystal mask marker of theraster irradiation type, as shown in FIG. 6B, one entire image P isdivided into five image segments Pa to Pf beforehand, and these dividedimage segments Pa to Pf are displayed by the liquid crystal mask 4successively. The deflector 5 is operated whenever each image segment isdisplayed, so that the divided image segments Pa to Pf are imprinted intheir respective optimum areas of the workpiece surface 6. After theentire image P has been imprinted (i.e., after the marking time T1), themarker system waits for a next workpiece (waiting time T2) and thenrepeats the above-explained marking operation on the next workpiece, asshown in FIG. 6A. While switching over to the next workpiece can beperformed by the deflector 5, a workpiece feeder 7 is employed for thatpurpose in this embodiment. Further, as shown in FIG. 5, the markersystem includes a controller 8, which is electrically connected to thelaser oscillator 1, the deflector 2 for raster irradiation, the liquidcrystal mask 4, the deflector 5, a drive motor 71 for the workpiecefeeder, and a workpiece position sensor 72, and which controls thesecomponents in synchronous relation in accordance with commands from anexternal terminal 9. The controller 8 also instructs application ofvoltages to the liquid crystal mask 4 (i.e., applies voltages to themask and makes it grounded), and adjustment of the applied voltages(i.e., raises and lowers the applied voltages).

It is to be noted that liquid crystal mask markers in which the presentinvention is employed are not necessarily limited to the liquid crystalmask marker of the raster irradiation type described above as anexample. Referring to FIG. 5, a liquid crystal mask marker of the lumpirradiation type is obtained by replacing the deflector 2 for rasterirradiation with a simple magnifying lens. Further, regardless ofwhether the marker is of the raster irradiation type or the lumpirradiation type, and regardless of whether the deflector 5 is providedor not, the liquid crystal mask marker can be designed to imprint anentire image as a whole rather than dividing the entire image into aplurality of image segments as explained above. In such a case, theprocedures shown in FIG. 6B are not required. Thus, it is convenient forunderstanding to regard "marking time T1" in FIG. 6A as "display" inFIG. 6B and "waiting time T2" in FIG. 6A as "grounded" plus "waitingtime".

In the first embodiment of the liquid crystal mask marker which can bepracticed in various forms, the control performed by the controller 8is, for example, as follows. The construction, operation and advantagesof the first embodiment will be described below with reference to FIG. 1in which there is shown an analytic graph. Note that the referencenumerals in FIG. 1 correspond to those in FIG. 4. When the display imageof the liquid crystal mask 4 is rewritten, the controller 8 outputssignals to the liquid crystal mask 4 to raise the voltage for the opaquepicture elements B of the current image P1 once to a value large enoughto make the picture elements B transparent, then ground or set all ofthe picture elements A and B equipotentially, and thereafter start thestep of displaying the next image P2. By raising the voltage for thepicture elements B to a value large enough to make them transparent, thehistory of each picture element B is cancelled. Here, the expression"raising the voltage for the picture elements B" does not mean that thetransmittance of the picture elements B is always increased to the samevalue as the transmittance of the picture elements A. Of course, if thevoltage for the picture elements B is raised to the same voltage as forthe picture elements A and then left to stand for a while, both thepicture elements A and B will have the same transmittance. Incidentally,it is not always required to raise the voltage for the picture elementsB to the same voltage as for the picture elements A.

With the first embodiment described above, as will be seen from the nextimage P2 in FIG. 1, the afterimage A2 remains, but the counterafterimage B1 is eliminated, resulting in a higher sharpness of theimage imprinted. In addition, since transmittance values of thetransparent areas A1 and B1 rise substantially at the same rate, thetiming to irradiate the laser beam is easier to set, and hence themarking speed can be increased with certainty.

Next, a second embodiment of the image display method for the liquidcrystal mask marker according to the present invention will bedescribed. This second embodiment is different from the first embodimentin the display method for rewriting the display image.

Referring to FIGS. 2 and 5, when the display image of the liquid crystalmask 4 is rewritten, the controller 8 outputs signals to the liquidcrystal mask 4 to lower the voltage for the transparent picture elementsA of the current image P1 once to a value low enough to make the pictureelements A opaque, then ground or set all of the picture elements A andB equipotentially, and thereafter start the step of displaying the nextimage P2. In this embodiment, at the timing at which the transmittanceof the picture elements A is reduced down to the same level as thetransmittance of the picture elements B, all of the picture elements Aand B are grounded, after which the voltages for displaying the nextimage P2 are raised. By so making all of the picture elements A and Bgrounded, the history of each of the transparent picture elements A andthe opaque picture elements B in the current image P1, i.e., the historyof the current image P1 itself, is cancelled. Note that, also in thisembodiment, it is not always required to lower the voltage for thepicture elements A to the same voltage as for the picture elements B.

With the second embodiment thus constituted, as will be seen from thenext image P2 in FIG. 2, the afterimage A2 and the counter afterimage B1are both eliminated, resulting in a higher sharpness of the imageimprinted. In addition, since transmittance values of the transparentareas A1 and B1 rise substantially at the same rate, the timing toirradiate the laser beam is easier to set, and hence the marking speedcan be increased. Incidentally, the prior art process of grounding thepotentials of all of the picture elements can also produce a similarcondition to that shown in FIG. 2 obtainable with this embodiment. Withthe prior art process, however, because a liquid crystal acts as onekind of capacitor, it takes a longer time to establish the samecondition as shown in FIG. 2 than with this embodiment. In short, thisembodiment can shorten a period of time required for establishing thecondition shown in FIG. 2.

Next, a third embodiment of the image display method for the liquidcrystal mask marker according to the present invention will bedescribed. This third embodiment is also different from the foregoingembodiments in the display method for rewriting the display image.

Referring to FIGS. 3 and 5, when the display image is rewritten, thecontroller 8 outputs signals to the liquid crystal mask 4 to lower thevoltage for the picture elements A of the current image P1 and raise thevoltage for the picture elements B thereof, then ground or set all ofthe picture elements A and B equipotentially, and thereafter start thestep of displaying the next image P2. By lowering the voltage for thepicture elements A and raising the voltage for the picture elements B,the current image is reversed. Preferably, the voltage for the pictureelements A is lowered to a level at which the picture elements A becomeopaque, and the voltage for the picture elements B is raised to a levelat which the picture elements B become transparent. Also, with all ofthe picture elements A and B grounded or set equipotentially, thehistory of each picture element in the current image P1 is cancelled.Note that the lowered voltage for the picture elements A is not alwaysrequired to be the same as the raised voltage for the picture elementsB. In this embodiment, when the voltages are lowered and raisedrespectively to such an extent that both the picture elements A and Bhave predetermined transmittance values, all of the picture elements aremade grounded, followed by starting the step of displaying the nextimage P2.

With the third embodiment thus constituted, as will be seen from thenext image P2 in FIG. 3, the afterimage A2 and the counter afterimage B1are both eliminated, resulting in a higher sharpness of the imageimprinted. Accordingly, the marking quality can be prevented fromdeteriorating at joints between divided images. This advantage ofpreventing a deterioration in the marking quality can also be attainedsimilarly in the above first and second embodiments as well. Inaddition, since transmittance values of the transparent areas A1 and B1rise substantially at the same rate, the timing to irradiate the laserbeam is easier to set, and hence the marking speed can be increased.

Three embodiments of the present invention have been described above indetail. Comparing the first to third embodiments, although resultingimages are different in sharpness to some extent from each other, any ofthe embodiments can raise the transmittance values of the transparentareas A1 and B1 substantially at the same rate. In other words, thetiming Ts to start irradiation of a high-density energy light beam canbe controlled, taking into account the conditions required to achieve ahigh marking speed and a high image sharpness. As a result, the markingspeed and the sharpness of an image imprinted can be both controlled ina desired manner. Additionally, it is a matter of course that thepresent invention is not limited to a liquid crystal mask marker with ahigh marking speed, but is also applicable to conventional liquidcrystal mask markers.

INDUSTRIAL APPLICABILITY

The present invention is usefully put into practice as an image displaymethod for a liquid crystal mask marker which can control the processingof an afterimage, the processing of a counter afterimage, and theirradiation timing of a high-density energy light beam, and which canalso display an image of a higher sharpness even when the marking speedis high.

We claim:
 1. In an image imprinting method of utilizing a liquid crystalmask having electrodes for a plurality of picture elements, said methodcomprising the steps of:(a) displaying on a liquid crystal mask a firstimage to be imprinted on a surface of a workpiece, said first imagehaving some picture elements which are transparent and some pictureelements which are opaque; (b) irradiating a light beam onto said liquidcrystal mask on which is displayed said first image; (c) irradiating alight beam, transmitted through said liquid crystal mask, onto saidworkpiece surface, thereby imprinting said first image on a workpiecesurface; (d) displaying on said liquid crystal mask a second image to beimprinted on a surface of a workpiece, said second image having somepicture elements which are transparent and some picture elements whichare opaque; (e) irradiating a light beam onto said liquid crystal maskon which is displayed said second image; (f) irradiating a light beam,transmitted through said liquid crystal mask, onto said workpiecesurface, thereby imprinting said second image on a workpiece surface;the improvement comprising: after step (c) and prior to step (d), thesteps of: (i) performing at least one of:(1) making all of the opaquepicture elements of the first image to become transparent, and (2)making all of the transparent picture elements of the first image tobecome opaque, (ii) then making the electrodes of all of the pictureelements of the first image to be grounded or set equipotentially, and(iii) thereafter starting step (d).
 2. A method in accordance with claim1, wherein said step of performing comprises making all of the opaquepicture elements of said first image to become transparent.
 3. A methodin accordance with claim 2, wherein the opaque picture elements of saidfirst image are made transparent by raising a voltage on the opaquepicture elements of said first image to a value large enough to makethem transparent.
 4. A method in accordance with claim 3, wherein thevalue to which the voltage on the opaque picture elements of said firstimage is raised is not the same as a voltage on the transparent pictureelements of said first image.
 5. A method in accordance with claim 3,wherein the value to which the voltage on the opaque picture elements ofsaid first image is raised is the same as a voltage on the transparentpicture elements of said first image.
 6. A method in accordance withclaim 2, wherein the opaque picture elements of said first image aremade transparent by raising a voltage on the opaque picture elements ofsaid first image to a value large enough to make them transparent.
 7. Amethod in accordance with claim 1, wherein said step of performingcomprises making all of the transparent picture elements of said firstimage to become opaque.
 8. A method in accordance with claim 7, whereinthe transparent picture elements of said first image are made opaque bylowering a voltage on the transparent picture elements of said firstimage to a value low enough to make them opaque.
 9. A method inaccordance with claim 8, wherein the value to which the voltage on thetransparent picture elements of said first image is lowered is not thesame as a voltage on the opaque picture elements of said first image.10. A method in accordance with claim 8, wherein the value to which thevoltage on the transparent picture elements of said first image islowered is the same as a voltage on the opaque picture elements of saidfirst image.
 11. A method in accordance with claim 7, wherein said stepof making the electrodes of all of the picture elements of the firstimage to be grounded or set equipotentially is started when atransmittance of the transparent picture elements of said first image isreduced to a level of transmittance of the opaque picture elements ofsaid first image.
 12. A method in accordance with claim 1, wherein saidstep of performing comprises making all of the opaque picture elementsof said first image to become transparent and making all of thetransparent picture elements of said first image to become opaque, tocreate a reversal of said first image.
 13. A method in accordance withclaim 12, wherein the opaque picture elements of said first image aremade transparent by raising a voltage on the opaque picture elements ofsaid first image to a value large enough to make them transparent, andwherein the transparent picture elements of said first image are madeopaque by lowering a voltage on the transparent picture elements of saidfirst image to a value low enough to make them opaque.
 14. A method inaccordance with claim 13, wherein the value to which the voltage on theopaque picture elements of said first image is raised is not the same asthe value to which the voltage on the transparent picture elements ofsaid first image is lowered.
 15. A method in accordance with claim 13,wherein the value to which the voltage on the opaque picture elements ofsaid first image is raised is the same as the value to which the voltageon the transparent picture elements of said first image is lowered. 16.A method in accordance with claim 12, wherein said step of making theelectrodes of all of the picture elements of the first image to begrounded or set equipotentially is started when a transmittance of thetransparent picture elements of said first image is reduced to a levelto which a transmittance of the opaque picture elements of said firstimage is raised.
 17. A method in accordance with claim 12, wherein saidstep of making the electrodes of all of the picture elements of thefirst image to be grounded or set equipotentially is started when atransmittance of the transparent picture elements of said first image isreduced to a predetermined level and a transmittance of the opaquepicture elements of said first image is raised to a predetermined level.18. A method in accordance with claim 1, wherein said liquid crystalmask is part of a liquid crystal mask marker employing rasterirradiation.
 19. A method in accordance with claim 1, wherein saidliquid crystal mask is part of a liquid crystal mask marker employinglump irradiation.
 20. A method in accordance with claim 1, wherein saidfirst and second images are separate segments of an entire image to beimprinted on a single workpiece.